Star Devours Companion

Artist's impression of a pulsar, approximately 20 kilometres in diameter, accreting material from a companion star. The strong gravity from the dense pulsar attracts material from the companion. The flow of gas from the companion to the pulsar is energetic and glows in X-ray light.Credit: NASA/Dana Berry

An international team of researchers has caught a star-turned-pulsar in the process of devouring its stellar neighbor, an act that accelerates its spin as it consumes more and more material.

The find, made using two space-based observatories, marks the first time astronomers have witnessed a pulsar speeding up it strips material from a companion star. [An animation of the process is available here via the European Space Agency (ESA).

Astronomers used the ESA's Integral gamma-ray observatory and NASA's Rossi X-ray Timing Explorer to observe the pulsar. The research will appear in an upcoming issue of the Astronomy and Astrophysics Journal.

"We're getting to the point where we can look at any fast-spinning, isolated pulsar and say, 'That guy used to have a companion,'" said Integral observation leader Maurizio Falanga, of Saclay, France's Commissariat ? l'Energie Atomique (CEA), in a statement.

The observations, researchers added, may help astronomers understand how slow-spinning pulsars in binary systems ramp up to become fast-spinning, solitary objects.

A pulsar is a spinning neutron star, which itself formed from the death of a massive star. Spinning neutron stars with strong magnetic fields can radiate energy like beacons as they spin, sending signals that appear as pulses - hence their name - to stationary observers.

While they have about the same mass as the sun, neutron stars are much smaller, about 12 miles wide (20 kilometers) or so wide. The Sun, for comparison, is about 865,000 miles (1.4 million kilometers) in diameter.

The pulsar studied by Falanga and his colleagues, known as IGR J00291+5934, is the sixth found to be feeding off a companion star, but the first observed while increasing its spin rate. The pulsar is one of the fastest on record, sending out an intense X-ray signal once every 1.67 milliseconds, researchers said.

The Integral observatory first recorded the pulsar when it flared from its position in the outer region of the Milky Way galaxy on Dec. 2, 2004. Researchers then used NASA's Rossi timing spacecraft to clock the pulsar's speed. While the Rossi spacecraft has found four of the six binary pulsar systems, the find was a first for Integral, ESA officials said.

Using the Rossi observations, researchers found that the pulsar had already reduced its companion star to a size much smaller than that of the sun, leaving it with only 40 times the mass of Jupiter. Locked in a binary orbit smaller than the radius of the sun, the two objects complete a full cycle every 2.5 hours and are close enough to allow stellar cannibalization - known as accretion - to occur.

But the process won't last forever. Eventually, the pulsar will completely devour its companion and be left to spin alone.

"Accretion is expected to cease after a billion...years or so," said Duncan Galloway, responsible for the Rossi observations at the Massachusetts Institute of Technology.